The present invention relates to a developing device for an electrophotographic image forming apparatus and, more particularly, to a developer carrier on which a magnet brush is selectively formed by a developer for developing a latent image electrostatically formed on the surface of a photoconductive element, or image carrier, and representative of a document image.
Electrophotographic image forming apparatuses such as a copier, facsimile transceiver and printer are extensively used today. In an electrophotographic copier, for example, a latent image representative of a document image is electrostatically formed on a photoconductive element and then developed by a toner or similar developer. For example, a two-component developer which is a mixture of toner and carrier forms a magnet brush on the surface of a developer carrier forming a part of a developing device and implemented as a developing roller. The magnet brush is caused into contact with the latent image. Then, the toner contained in the developer is selectively deposited on the latent image on the basis of the potential pattern of the latent image. Usually, a bias for development is applied to the developing roller. A toner image produced by the above procedure is transferred to a paper sheet or similar medium by a transferring device and then fixed on the medium by a fixing device of the type using a heater by way of example.
Assume that a conductive base constituting the photoconductive element is exposed to the outside due to scratches or similar defects formed in the surface of the photoconductive element. Then, the magnet brush deposited on the developing roller makes contact with the exposed base of the photoconductive element and thereby causes the bias being applied to the roller to leak, resulting in defective reproductions.
Providing an insulating layer on the surface of the developing roller is an approach proposed in the past for eliminating the above problem. Although this kind of approach is successful in eliminating the leak of the bias, it brings about another problem since the insulating layer is charged due to the friction thereof with the developer. Specifically, a color electronic copier or similar image forming apparatus usually has a developing device which is made up of two or more developing units. In this type of apparatus, while one of the developing units is in operation, the others are held in an inoperative condition. Therefore, a prerequisite is that the magnet brush be formed only on the developing roller of the operative developing unit and not on the developing rollers of the inoperative developing units. However, once the insulating layer provided on the developing roller for the above purpose is charged due to its friction with the developer, the developer is electrostatically deposited on the charged insulated layer. As a result, the magnet brush is unwantedly formed on the developing rollers of the inoperative developing units. For this reason, it has been customary to simply replace the defective photoconductive element or to repair it in place of adopting the insulating layer scheme.